"As with ported enclosures, PR enclosures are lined but not stuffed. Although not always suitable as a general-purpose port replacement, PR's offer some unique advantages of their own, especially in the frequency range around and below the tuning frequency. "

As you know, a PR is essentially a bass reflex alignment, substituing one type of oscillator(vent) for another one(drone cone).

As in a bass reflex alignment...adding stuffing increases 'effective volume', but this is not entirely accurate as would be with a sealed alignment. As you increase stuffing, and lower Fb, you in turn effect efficiency proportinately, eventually negating the benefits of increased efficiency of the 4th order alignment in the first place!. IMO lining and/or fill should ONLY be used in a ported/pr alignment to reduce internal box reflections, etc. For this reason, it is of dire importance to choose a box shape that exhibits minimal standing wave gain and backwave reflection in the first place when using a pr/vented box with a midbass driver.